The present invention relates to the strengthening of sintered ferritic stainless steels. Such steels are useful in demanding automotive applications such as flanges for exhaust systems.
Powder metallurgy (P/M parts are made by pressing metal (or alloy) powders into a compact, followed by sintering the compact at a high temperature in a protective atmosphere. P/M stainless steel parts are commonly made by using pre-alloyed powders of the desired composition. Water-atomized pre-alloyed, minus 100 mesh powders are typically used, since these offer good green strength and compressibility and are cost effective. Although fully pre-alloyed powders are commonly used, the powder metallurgy process is amenable to the use of additives for the enhancement of properties of the sintered parts. The high sintering temperatures (above ca. 2000.degree. F.) and long sintering times (&gt;20 minutes) employed are in most instances sufficient for substantial diffusion and alloying of the additive metal in the matrix alloy.
P/M stainless steel parts offer cost advantages over their wrought counterparts, while maintaining the requisite mechanical strength, corrosion resistance, oxidation resistance and elevated strength. The P/M process is quite flexible and allows enhancement of one or more critical properties for a given application by making only minor modifications in the alloy composition, use of additives and/or changes in processing parameters.
In some applications, however, the strength of P/M stainless steel parts may not be sufficient. Specific examples are the flanges used in automobile exhaust systems. These flanges are either welded or bolted onto the engine or onto other components of the exhaust system. Important properties for such flanges include corrosion resistance, oxidation resistance, mechanical strength and impact resistance, at both ambient and elevated temperatures. High strength is essential for maintaining the leak-tightness of the flange-to-flange and flange-to-manifold bolted joints, so that the exhaust gases do not leak out of the exhaust system prior to entering the catalytic converter. Wrought stainless steel flanges perform satisfactorily, in general; however, the geometry and sizes of these flanges are such that the P/M process would be significantly less costly. The P/M process also offers more flexibility with the design of the flanges, permitting the selection of the optimum design for the best performance and weight control for specific locations and various automobile models.
Ferritic grades of stainless steels are almost always used in automobile exhaust systems for flanges, pipes, HEGO (Hot Exhaust Gas Oxygen Analyzer) bosses and other components. These grades of stainless steel are cost effective and offer adequate corrosion resistance, oxidation resistance and mechanical strength.
Ferritic stainless steels, however, are generally not heat treated because they do not undergo phase transformations that increase strength and hardness after heating and fast cooling. (Martensitic alloys, on the other band, can be hardened by heat treatment.) If an application, therefore, requires sintered ferritic stainless steels of higher strength, such added strength is usually achieved by increasing the sintered density or increasing the alloy content. For example, the commonly used ferritic P/M stainless steels are AISI types 409L, 410L, 430L and 434L; the strength increase associated with the change from the low alloyed 409L to the higher alloyed 434L is in the range of about 10 to 15 percent when expressed in terms of ultimate tensile strength (UTS). In some instances, such an increase may not be sufficient and, additionally, the higher alloyed grades cost more.
P/M stainless steels may also be sintered in an atmosphere of dissociated ammonia, in which case the steels absorb substantial amounts of nitrogen which provide significant solid solution strengthening. Without rapid cooling after sintering, however, corrosion resistance will be drastically reduced due to sensitization. Acceptable cooling rates are several hundred degrees C per minute, which are not commercially feasible at the present state of the art of sintering. Thus, this method of strengthening is generally not practiced when corrosion resistance is important.
In the area of wrought ferritic stainless steels, U.S. Pat. No. 2,210,341 discloses a nickel addition of 0.3 to 3% to welding rods containing from 8 to 15% Cr, 0.3 to 3% Mn, 0.3 to 3% Mo and 0.02 to 0.07% carbon, with the balance iron. The addition of nickel promotes a fine grain structure and makes the welds tough and ductile. Some of the more recent wrought ferritic stainless steels contain small amounts of nickel because of its beneficial effect on toughness, on lowering the ductile-to-brittle transition temperature, and on improving their passivity characteristics. P/M stainless steels do not undergo grain growth as the wrought stainless steels do, and hence do not require nickel addition to control grain structure. Even with the wrought ferritic stainless steels, nickel addition is much less frequently practiced due to the advent of nickel containing welding wires which can provide nickel to the weld zone.
Accordingly, it is desirable to increase the strength of sintered ferritic stainless steels without requiring rapid post-sintering cooling and without reducing corrosion resistance. An object of this invention is to produce sintered ferritic stainless steel compositions having such properties. Another object is to produce sintering powders comprising ferritic stainless powders containing nickel as a pre-alloyed and/or blended powder component.